Generalized Thermoelastic Coupling Problem Of Ice And Components In PEMFC At Quick Cold Start-Up

The non-classical heat transfer effect caused by rapid cold start of vehicle proton exchange membrane fuel cell poses a new challenge to heat transfer analysis.Firstly,as a material with structural phase change at a single temperature,the melting problem of ice is essentially the heat conduction problem of moving interface with phase change latent heat,and the effect of high-speed thermal shock will make its heat transfer and deformation more complex.Secondly,non-Fourier thermal deformation is often accompanied by obvious thermoelastic coupling effect,that is,the instantaneous local extreme deformation caused by the non-uniform distribution of heat in turn affects the heat conduction process.The strong coupling characteristic between deformation and heat conduction is difficult to be accurately described and characterized by classical thermoelastic mechanics and the existing generalized thermoelastic theory,and the simple decoupling treatment will make the theoretical prediction biased,Therefore,considering the changes of material phase transition and expansion rate of the host material to the mathematical characterization and mechanism characterization of the generalized thermoelastic coupling model is also of great research value.Finally,the interface between ice and components is interrelated,elastically constrained,and subjected to thermal shock at the same time.The problem model of the coupling of two physical fields and two deformation fields is also the key basic theory to accurately predict the thermal shock response of frozen components in the process of extreme ice melting.Based on the above three points,the main work of this paper is as follows:1.Firstly,based on the traditional Fourier heat conduction theory and the Cattaneo-Vernotte heat conduction theory considering the hysteresis effect of heat flow change,the heat conduction model of spherical ice particles under constant temperature thermal shock is established.Combined with the mechanical equation considering thermal stress,the non-Fourier effect of transient heat conduction process is studied,and the significance of using non-Fourier heat conduction model in the low-temperature rapid start-up of proton exchange membrane fuel cell is discussed,The effects of thermal relaxation time,model size and load temperature on temperature distribution,stress response and ice melting rate are analyzed.The results show that the non-Fourier characteristics of thermal shock response are quite significant under the calculation conditions in this paper,and the serious uneven distribution of temperature and stress increases the risk of material damage.The thermal relaxation time and model size are positively correlated with the ice melting time,while the temperature load is negatively correlated,and there is a limit.The pursuit of extreme ice melting is essentially the balance between the ice melting time and the stress level.2.According to the freezing and melting characteristics of ice materials,three classical methods for solving the moving interface heat conduction problem with phase change latent heat are selected:enthalpy method,sensible heat capacity method and phase interface tracking method.By introducing the additional phase lag term of heat flow into the global heat conduction control equation and phase interface energy balance equation respectively,three kinds of transient heat conduction models with phase change are established,The transient heat conduction model of phase interface tracing method,which is more accurate in the calculation of phase transformation process,is selected to analyze the thermal shock response of ice particles under constant heat load.The results show that the non-Fourier transformation of global and phase interface heat transfer control equations will bring the transmission and superposition of non-Fourier effect,and aggravate the uneven distribution of temperature and stress.The size change of ice particles is essentially the result of the joint action of material phase transformation and thermal expansion,and many parameters will affect the coupling effect of the two to varying degrees.The initial melting velocity cannot be determined by a single parameter,especially if the thermal relaxation time is too large or the particle size is too small,the size change of ice particles will always be dominated by the phase velocity.3.Aiming at the thermoelastic coupling effect in the process of extremely fast ice melting,based on the phase interface tracking method with phase change transient heat conduction equation,the influence of the local deformation rate of the host material on the heat transfer process is further considered.By dividing the heat flux into two parts:the conduction flux caused by the temperature gradient and the convection flux determined by the deformation velocity field and the temperature field,the phase change transient heat conduction equation with the local deformation rate of the host material is established,The description of the generalized thermoelastic coupling characteristics is improved.The results of the coupled partial differential equation of free expansion of ice particles under constant temperature load are compared with the classical Lord Shulman generalized thermoelastic theory and uncoupled model.The strong thermal coupling effect in the thermal shock problem with phase change is analyzed and compared from the perspectives of expansion rate and strain energy of the host material.The results show that the expansion process always hinders the inward conduction of heat.In the initial stage of heat transfer,it will aggravate the heat accumulation on the surface,resulting in the increase of stress amplitude.In the later stage,it will slow down the temperature rise,hinder the intensification of deformation and stress rise,which also reflects the strong coupling effect of heat transfer and deformation process in the thermal shock problem.In contrast,under the calculation conditions in this paper,the coupling term in L-S model only has a certain inhibitory effect on the stress fluctuation at the initial stage of heat transfer,and the influence is relatively small.4.After constructing the transient thermoelastic coupling characterization system with phase transition based on phase interface tracking method,this theoretical model is tried to be applied to the actual working condition of PEMFC rapid cold start.According to the material and structural characteristics of bipolar plate,gas diffusion layer and catalytic layer in PEMFC,the component ice water core-shell model is simplified to simulate the extreme speed ice melting process under constant temperature load.By analyzing the influence of thermal shock process and ice particle deformation on the structural integrity of the component,the theoretical critical temperature load allowed to be applied by each component in the ice melting process is given.The results show that under the combined action of thermal shock and ice core expansion,the vulnerable parts of the three components are located on the inner surface of the spherical shell,and their maximum critical loads are f_0BP=1354K,f_0GDL=617K and f_0CL=1656K respectively.Among them,the thermal conductivity of graphite bipolar plate is high,and the energy consumption for ice melting is large;The spherical shell of gas diffusion layer is too thin to effectively inhibit the expansion and deformation of ice core,which is easy to lead to material cracking;In contrast,the catalytic layer is difficult to be damaged due to its high specific heat capacity,low elastic modulus and small pore size.The main features and innovations of this paper are as follows:1.It is proposed for the first time that the non-Fourier effect of thermal shock response of icing components should be considered in the analysis of rapid cold start of PEMFC,and the necessity of using non-Fourier heat conduction theory to analyze the problem is demonstrated from the aspects of thermal relaxation time,model size and temperature load.2.Considering the characteristics of ice materials,the finite propagation of thermal disturbance is considered in the heat conduction model of moving interface with phase transition for the first time,and a heat conduction equation more suitable for describing the problem of extreme ice melting is established.3.The convective heat flux of deformation is introduced into the transient heat conduction equation,the description of thermal mechanical strong coupling in the theoretical model is improved,and a complete transient thermoelastic coupling characterization system with phase transition is constructed.4.The new theoretical method is effectively applied to practical engineering problems,and the maximum critical load of rapid ice melting of each core component is explored theoretically,which provides a theoretical basis for PEMFC rapid cold start control.